This invention relates to environmental air conditioning systems (ECS), and more specifically to air cycle environmental air conditioning systems such as used on aircraft.
Aircraft that fly at altitudes above that at which ambient air is suitable for the health and comfort of crew and passengers are often equipped with air cycle environmental air conditioning systems. Such systems provide pressurized conditioned air or for cooling passengers, crew, and other aircraft systems and components. These air conditioning systems typically use high pressure air bled from a turbine engine or auxiliary power unit (APU). In some cases high pressure air may be provided by an electrically-powered compressor. The high pressure compressed air fed into these systems typically is at a temperature and pressure far in excess of the temperature and pressure required for conditioned air to be supplied to the cockpit and passenger cabin, so it must be expanded and cooled by the air conditioning system before it can be discharged into the aircraft cabin as conditioned air. A typical prior art ECS system is depicted in FIG. 1.
As shown in FIG. 1, in a typical environmental air conditioning system, compressed air 112 from a compressed air source (not shown) such as a turbine engine bleed or an APU bleed is cooled in a pre-heat exchanger (not shown) (also referred to in the art as a precooler heat exchanger), where it is cooled to a temperature suitable for delivery to an ECS pack 100 that is typically located in the wing of the aircraft near the fuselage. The flow of air to the ECS pack is regulated by valve 114 and directed through conduit 116 to a heat exchanger 115 (also referred to in the art as a primary heat exchanger) where heat is rejected to ambient air flowing through or across a heat absorption side of heat exchanger 115. Cooled compressed air is discharged from heat exchanger 115 to compressor 120. A portion of the air going to heat exchanger 115 can be controllably diverted through conduit 117 and control valve 119 to mix with the outlet of turbine 144 to control the temperature of the conditioned air. Compressor 120 compresses its portion of the air from heat exchanger 115, which also results in heating of the air. The further compressed air is discharged from compressor 120 through conduit 124 to heat exchanger 126 (also referred to in the art as a secondary heat exchanger) where it rejects heat to ambient air flowing through or across a heat absorption side of heat exchanger 126.
The ambient air 113 flowing through or across the heat absorption sides of heat exchangers 115 and 126 can be a ram air flow circuit from a forward-facing inlet of the aircraft. In conditions under which insufficient airflow is generated by the forward motion of the aircraft for cooling of heat exchangers 115 and 126, the air flow can be assisted by operation of fan 128. Check/bypass valve 129 allows for bypass of the fan 128 when ram air flow is sufficient for the cooling needs of heat exchangers 115 and 126. Heat exchangers 115 and 126 can share a flow path for the ambient cooling air in either a parallel or series configuration, and can be integrated into a single unit with heat exchanger 115 sometimes referred to as a primary heat exchanger and heat exchanger 126 sometimes referred to as a secondary heat exchanger. Cooled air discharged from heat exchanger 126 is delivered through conduit 132 to a heat rejection side of heat exchanger 130. In the heat rejection side of heat exchanger 130, the air is further cooled to a temperature at or below the dew point of the air and flows into water removal unit 135 where liquid water 136 condensed from the air is removed. The dehumidified air flows through a heat absorption side of heat exchanger 130 where it is re-heated before being delivered through conduit 138 to turbine 140, where work is extracted as the air is expanded and cooled by turbine 140. A portion of the air going to turbine 140 can be diverted by valve 141 if needed to allow the temperature of the air at the inlet to the heat absorption side of heat exchanger 130 to be above freezing. The cooled expanded air discharged from turbine 140 is delivered through conduit 142 to the heat absorption side of heat exchanger 130 where it provides cooling needed to condense water vapor from the air on the heat rejection side of heat exchanger 130. The air streams on the heat absorption side of heat exchanger 130 are thus reheated. Heat exchanger 130 is also sometimes referred to as a condenser/reheater, and can be integrated with water removal unit 135 in a single unit. The reheated air from conduit 142 exiting from the heat absorption side of heat exchanger 130 flows through conduit 143 to turbine 144, where it is expanded and cooled, and then discharged from the system 100 through conduit 145 to mix manifold 150 where it is mixed with recirculated cabin air 152 before being discharged to the aircraft cabin. The environment air conditioning system 100 also includes a power transfer path 147 such as a rotating shaft that transfers power to the compressor 120 and fan 128 from work extracted by turbines 140 and 144.
Prior art ECS systems such as described above rely on centralized redundant ECS packs typically located in each wing root of the aircraft near the fuselage. These systems have proven effective and reliable, but they are relatively complex systems, which drives up cost and adversely affects reliability. Additionally, the use of a ram air circuit for heat rejection adds to aircraft aerodynamic drag and takes up space on the aircraft. Also, cabin air recirculation fans and recirculation ducting take up significant space, and the need for trim air systems to manage variable thermal loads adds to control complexity and takes up more space on the aircraft.